Worldwide, toxoplasmosis and other infections caused by obligate intracellular parasitic agents remain a major cause of morbidity and mortality in humans and in livestock. A central mediator of host defense against these agents is IFN-gamma, a cytokine produced by Type 1 lymphocytes and NK cells. Toxoplasma gondii infection in the mouse represents a well-characterized model to study the differentiation of type 1 cells and a highly stringent test to assess IFN-gamma function in vivo. This proposal will systematically define the intercellular and intracellular signals that govern the prompt initiation, maintenance and successful functioning of IFN-gamma-mediated responses critical for host survival. The experimental methods rely on in vivo infection and genetic studies of normal and defective mouse strains aided by ex vivo cellular and molecular analyses of lymphocyte function. The first aim will critically assess the lymphocyte cell lineage and transcription factor requirements for IL-12 induced, IFN-gamma mediated host resistance to T. gondii infection. The second aim evaluates the role of Tbet, a novel Thi -specific transcription factor in sustaining long term IFN-gamma effector function and thus immune protection. A final third aim will provide detailed phenotypic and genetic analyses of a newly discovered defect in early IL-12 induced IFN-gamma responses resulting in acute susceptibility to T. gondii infection. The influence of this genetically dictated, partial lL-12 unresponsiveness on signal transduction and transcription factor(s) pathways critical for IFN-gamma production will be evaluated. The proposed experiments should provide novel information on cellular and molecular regulatory determinant(s) affecting the generation, maintenance and appropriate functioning of IFN-gamma producing type 1 lymphocytes during a successful and balanced immune response. Since many infectious and autoimmune pathologies result from Type 1 effector dysfunction, the insights provided by the proposed studies should eventually result in improved management of such disease states. This grant should also provide insights for designing immunization strategies to induce sustained type 1 responses.